The present invention relates to point to multipoint networks and more particularly to systems and methods for sharing available bandwidth efficiently among multiple users.
Point to multipoint networks are increasingly being deployed to provide end user access to the Internet. One example of a point to multipoint network is a data over cable network. In such a network, an existing CATV infrastructure previously used for distributing cable broadcast signals is modified to provide two-way data communication capability and Internet access. A cable head end acts as a central access point and exchanges information with subscriber units located at residences and/or businesses.
Another example of such a point to multipoint network is a fixed wireless access network. The architecture is similar, only now the central access point and multiple subscriber units exchange information using radio waves.
Either type of point to multipoint network may employ frequency division duplexing (FDD) where downstream transmissions (central access point to subscriber unit) use different frequency(s) than upstream transmissions (subscriber unit to central access point). The central access point is then the only entity that transmits on the available downstream frequencies.
Upstream bandwidth is, however, shared among multiple subscriber units. One typical scheme for sharing this upstream bandwidth is time division multiple access (TDMA) where the time domain is divided into multiple slots and only one subscriber unit may transmit successfully at a time. Access to individual time slots may be determined by either reservation where the central access point assigns time slots or by contention where subscriber units may attempt to transmit during a particular time slot without reservation. In the contention scheme, if there is a collision, the conflicting subscriber units wait a psuedo random interval before attempting retransmission. A combination of contention and reservation may also be used.
An example of a point to multipoint network protocol based on TDMA and combination of contention and reservation techniques is the DOCSIS v1.1 standard described in the Data-Over-Cable Interface Specifications, Radio Frequency Interface Specification SP-RFIv1.1-I06-001215 published by Cable Television Laboratories in 2000, incorporated herein by reference in its entirety and herein referred to as “DOCSIS v1.1 Specifications” or simply “DOCSIS v1.1”. This DOCSIS specification is finding application in both the data over cable and wireless environments, although wireless environments may advantageously substitute for the physical layer portion of DOCSIS v1.1 to better handle the challenges presented by wireless channels.
The broadband Internet access provided by point to multipoint networks is increasingly viewed as a key enabler for streaming data services such as voice and video. If properly configured, the point to multipoint network can provide services such as local and long distance telephony as well as video on demand. The characteristics of this type of traffic makes special demands on the MAC (media access contention) layer protocol employed by the point to multipoint network. For example, the MAC layer portion of the DOCSIS 1.1 protocol makes special provisions for voice calls.
To support streaming applications, DOCSIS provides for multiple “service flows” where each service flow is a unidirectional flow of packets that is provided a particular quality of service. A voice call can be assigned a service flow which will then be handled by the MAC layer to provide minimal latency while maintaining optimal bandwidth allocation among multiple subscriber units. For example, a voice telephony call may be assigned to a service flow that is supported by an “unsolicited grant service.” In an unsolicited grant service, a subscriber unit supporting a voice call need not request a reservation for each upstream slot that it needs to transmit voice data. Instead, the central access point gives the subscriber unit fixed sized grants at regular intervals. This is more efficient because the upstream bandwidth is then not cluttered with access requests for each and every slot needed to transmit the streaming data.
The use of this unsolicited grant service, however, may create another potential inefficiency. Voice calls are often full of quiet periods where the speaker is silent and there is no data to transmit. Many of the slots then allocated by the unsolicited grant service would be wasted. To address this concern and increase system efficiency, DOCSIS v1.1 also provides for an unsolicited grant service with activity detection where the subscriber unit can detect a pause in the voice data stream and requests suspension of the unsolicited grants. The slots thus freed up then become available for use by other traffic.
To resume upstream transmission of voice traffic after a quiet period, subscriber unit must again request access. To accommodate these access requests, the central access point provides periodic access request slots that are allocated exclusively to a particular subscriber unit that has an inactive service flow. This is in contrast to the access request technique that DOCSIS v1.1 provides for non-real time traffic in which subscriber units contend for access request slots. The motivation for providing exclusive allocations to access request slots rather than relying on a contention mechanism here is that if a request for resumption of the unsolicited grant service were to collide, data slots would not be made available soon enough to avoid loss of voice data. This effect would directly impact call quality.
A problem arises, however, in that these dedicated access request slots that are allocated to inactive service flows use bandwidth that is then unavailable for other services. Each inactive call supported by the network adds to this access request overhead since the subscriber units do not share these access request slots. What is needed is a more efficient activity resumption mechanism for service flows in a point to multipoint network.